为何Unix是‘自支撑’系统？前Unix系统在哪些方面不具备该特性？

Great question! When Ritchie and Thompson highlighted that Unix was fully self-supporting, they were pointing out a radical shift from the computing systems that came before it. Most pre-Unix systems lacked this ability, and here’s how that played out in practice:

• 依赖外部/专属开发环境
  Early systems like IBM’s OS/360 or DEC’s TOPS-10 relied heavily on specialized, hardware-tied tools that didn’t run on the system itself. For example, compilers or kernel modifications often had to be built on a separate large mainframe, then transferred to the target system via physical media (like tape). You couldn’t write, compile, and test system-level code entirely within the system’s own environment.

• "宿主-目标"分层开发模式
  Many minicomputer and early microcomputer systems (think CP/M) used a "host-target" workflow. Development tools (assemblers, high-level language compilers) ran on a more powerful host system (often a mainframe or larger minicomputer), generated executable code for the target system, and then that code was loaded onto the target via a bootloader. The target system couldn’t bootstrap itself—you couldn’t use the system’s own tools to rebuild its core software.

• 缺乏统一的用户空间工具链
  Pre-Unix systems didn’t have a cohesive set of user-space tools like Unix’s shell, text editors, and file utilities. System administration, code editing, and even basic file operations often required using proprietary console commands or hardware-specific interfaces. There was no way to use the system’s own tools to maintain or extend the system itself; you’d need vendor-provided specialized software or hardware debuggers.

• 硬件与软件深度耦合
  Most pre-Unix systems were tightly bound to their specific hardware. Kernel components and system tools were tailored exclusively for the machine’s architecture, with no abstraction layer. This meant you couldn’t easily port the system to similar hardware, and modifying the system to work with new hardware required vendor-specific tools that didn’t run on the system itself.

To tie back to the quote from Ritchie and Thompson:

值得注意的是，该系统完全是自支撑的

This was groundbreaking because Unix broke these cycles—its compilers, shell, editors, and even kernel build tools all ran on Unix itself. You could bootstrap the entire system from a minimal bootloader, then use Unix’s own tools to build and maintain every part of it.

内容的提问来源于stack exchange，提问作者aupreti